The compressive load carrying capacity of a layer of resilient material may be increased several hundred percent by incorporating a plurality of spaced, parallel laminae fabricated nonextensible material and oriented generally perpendicular to the direction of the anticipated compressive load. The laminae increase the compressive load carrying capacity of the resilient material by reducing the ability of the material to deflect or bulge in directions transverse to the direction of the compressive load. At the same time, the ability of the resilient material to yield in shear or in torsion in directions parallel to the laminations or transverse to the direction of the compressive load is substantially unaffected. The characteristics of such laminated resilient material have resulted in the commercial acceptance for a variety of applications of bearings incorporating laminated material. One area of particular importance is the mounting of helicopter rotor blades on an associated rotor hub.
In some mechanical designs, a bearing must be able to deflect in response to rotational loads or forces and be able to support, without significant deflection, compressive loads applied in a single, known direction. A cylindrical, laminated resilient bearing can provide the required characteristics. Within such a cylindrical laminated bearing, the individual layers or laminations may be either disc-like or tubular in shape. A laminated bearing with disc-like laminations can resist or support compressive loads applied parallel to the axis of rotational loading, which passes through the centers of the discs. With tubular laminations, a laminated bearing can resist or support loads applied radially or perpendicularly relative to the axis of rotational loading.
In other mechanical designs, a bearing must be able to resist or support compressive loads applied in two or more generally perpendicular directions, while also being able to deflect in response to rotational loads. Such requirements can be met by a laminated resilient bearing that is frustroconical in shape and has layers or laminations which are also frustroconical in shape. (Such a frustroconically shaped bearing is to be distinguished from a laminated bearing which has an overall frustroconical shape, but in which the layers or laminations are tubular, for example, and are merely displaced axially relative to one another in order to provide the overall frustroconical shape of the bearing.) The use of a frustroconically shaped laminated bearing to resist or support compressive loads applied in two or more generally perpendicular directions, while being free to deflect in response to rotational loads, is shown in patents such as: Hinks U.S. Pat. No. 2,900,182; Krotz U.S. Pat. No. 3,179,400; Cresap et al U.S. Pat. No. 3,652,185; and Gorndt et al U.S. Pat. No. 3,862,812. A method of manufacturing a frustroconical laminated bearing is described and illustrated in Galbato U.S. Pat. No. 3,503,820.
When frustroconically shaped laminated bearings are utilized in automobile suspensions, such as are described and illustrated in the Hinks and Krotz patents, for example, the bearings are subjected to moderate compressive loads. When used to mount helicopter rotor blades to a rotor hub, as described and illustrated in the Cresap et al and Gorndt et al patents, on the other hand, the laminated frustroconical bearings are subjected to significantly more severe compressive loads, in the range of 50,000 to 100,000 pounds. To accommodate the higher compressive loads, one alternative is to build larger, more massive bearings. Such an approach becomes less and less acceptable, however, as the technology involved in the construction of helicopters advances and produces increased demands, in terms of still higher loads and prolonged service life, on bearings used in helicopters. In addition, efforts to improve fuel economy, for example, have increased the desirability of reducing the size and weight of all components of a helicopter.
As the operating loads have increased on laminated bearings utilized in helicopter rotor blade retention systems, it has become increasingly common to see a type of bearing failure in which the resilient or elastomeric material extrudes from between adjacent rigid or nonextensible laminations. In investigating such bearing failures, it has been found that when large compressive loads are applied to a laminated bearing, unusually large shear strains may be produced at the edges of the layers of resilient or elastomeric material. The shear strains result from unexpectedly high, localized compression of the resilient material by and between the nonextensible or rigid layers of the bearing. Physically, the strains in the resilient or elastomeric laminations are first apparent as bulging of material from between the nonextensible laminations. The bulging exposes the resilient material to frictional wear or fretting and, ultimately, results in large-scale extrusion from between the nonextensible laminations. The problem of excessive bulging or compression induced shear strain in a laminated resilient bearing has been recognized, and a solution proposed, in copending U.S. patent application Ser. No. 632,423, filed Nov. 17, 1975, now U.S. Pat. No. 4,040,690 which is assigned to the assignee of the present application.
The problem of fatigue failure due to compression induced shear strains may occur in a frustroconically shaped laminated bearing, just as in any other laminated bearing. It has been found, however, that the problem becomes more severe and appears at lower loads when certain steps are taken to reduce the size, and hence the weight, of a frustroconical laminated bearing. More particularly, one approach to reducing the space that must be provided to accommodate a frustroconically shaped laminated bearing is to reduce the outer circumference of the bearing. The outer circumference of the bearing can be reduced making each frustroconical lamination the same size, with the same maximum diameter or outer circumference. Such construction is to be contrasted with the alternative construction, which is to fabricate successive laminations with increasingly larger diameters so that each lamination is more fully nested in an adjacent lamination. The difference between the two construction techniques can be seen by comparing the frustroconically shaped bearing shown in FIG. 3 of Hinks U.S. Pat. No. 2,900,182 with the frustroconically shaped laminated bearing shown in FIG. 3 of Krotz U.S. Pat. No. 3,179,400 or in FIG. 2 of Galbato U.S. Pat. No. 3,503,820. As will be explained below, a frustroconical bearing fabricated of laminations all having the same maximum diameter is more likely and sooner likely to experience fatigue failure due to compression induced shear strains.
In the frustroconical bearing of Hinks U.S. Pat. No. 2,900,182, for example, the larger ends of the individual layers or laminations together define an annular surface of the bearing. The annular bearing surface is oriented normal or perpendicular to the frustroconically shaped side surfaces of the laminations. In the frustroconical laminated bearing of Krotz U.S. Pat. No. 3,179,400, on the other hand, the corresponding annular surface of the bearing forms an angle of about 30.degree. to 50.degree. with respect to the frustroconical side surfaces of the individual layers or laminations in the bearing. What could be regarded as an end surface of the Hinks bearing is essentially a side surface of the Krotz bearing. A frustroconically shaped laminated bearing that may incorporate a design or construction technique intermediate the designs or techniques found in the Hinks and Krotz bearings is shown in FIG. 4 of Cresap et al U.S. Pat. No. 3,652,185. The Cresap et al patent does not discuss the angular orientation between the side surfaces of its bearing layers and the annular surface defined by the larger diameter ends of the layers. Nonetheless, direct measurement of the patent drawings indicates an angle of about 75.5.degree.. Stated another way, the surface of the bearing formed by the larger diameter ends of the laminations appears to be disposed, when viewed in radial section, at an angle of about 14.5.degree. with respect to a line that is oriented normal to the frustroconical side surfaces of the layers or laminations in the bearing.
The adverse effects on the fatigue life of a frustroconical laminated bearing which result from efforts to reduce the overall size of the bearing were discovered during a program to design a bearing suitable for use in a helicopter rotor blade retention system similar to that shown in the Cresap et al patent. To reduce the outer circumference and overall size of the bearing, the surface of the bearing defined by the larger diameter ends of the laminations was "swept back" from an orientation normal to the side surfaces of the laminations in the bearing. A sample bearing was constructed in which the surface of the bearing defined by the larger diameter ends of the bearing laminations formed a "sweep angle" of approximately 30.degree., when viewed in radial section, with respect to a line normal to the frustroconical side surfaces of the layers or laminations in the bearing. Upon testing, the sample bearing with the 30.degree. sweep angle failed prematurely and was not considered commercially acceptable in comparison to a similar frustroconically shaped laminated bearing in which the sweep angle was 0.degree. (i.e., the surface defined by the larger diameter ends of the layers was oriented normal to the frustroconical side surfaces of the bearing layers or laminations). As a result, a frustroconical laminated bearing with a sweep angle of 0.degree. was proposed for use in the blade retention system.